published May 6, 2020 seacoastonline.com, the York Weekly, Portsmouth Herald, Foster’s Daily and the York County Coast Star
The last time I wrote about water striders, it was the middle of the summer. I was sitting by a pond battling mosquitoes while watching them skate across the surface of the pond. I love how fast they move, which I didn’t understand. Are they skating and digging in their little feet for some purchase on the slick surface of the pond? Or is it a sticky surface that just looks like glass?
Turns out not all small insects can do this – walk and skate on water. Water striders can because they have very fine hairs on the undersides of their legs that trap air and repel water. The scientific term for this is superhydrophobic. They can move so quickly because what they are doing is more like rowing, vigorously rowing, creating little swirls in the surface that help propel them forward. For their body size, they move fast, the equivalent of a 6-foot-tall person running 400 miles per hour!
I love the way their feet make little dimples in the surface of the water. Sometimes that’s how I first notice them – by the shadows those dimples cast on the bottom of the stream. As a biology teacher, I really love this, a textbook example of the high surface tension of water. They are bending the surface of the water.
I have been surprised to see water striders on my brook and along the edges of the river. I hadn’t realized they lived in flowing water as well as still water. Having never lived along a river until now, I have always made assumptions about who lives where. This was a big one. I always assumed they needed still-water, but there they were, skating upstream against the current, hanging out in the still water along the edges. And, as a wonderful sign of spring, this past weekend, while I was battling newly-hatched black flies instead of mosquitoes, I was able to catch some in the act of reproduction.
I realized these two were mating because they looked huge and on closer inspection it turned out I was seeing two, one being carried on the others’ back. So, I looked into water strider mating behavior. As you would expect, it is fascinating. As far as researchers know, there is no courtship involved. The male mounts the female. If she doesn’t fancy him, she might try to resist by deploying an extremely effective genital shield. However, the males have coevolved a behavior to prevent her from resisting, an extremely diabolical behavior. They coerce the female into mating by tapping out intricate patterns on the surface of the water. These patterns are meant to attract a predatory beetle that attacks from below the surface, the backswimmer. The female, since she is on the bottom, is more vulnerable to attack from below, so usually submits fairly quickly if the male starts tapping. This has been tested in an experiment (Han and Jablonski, “Male water striders attract predators to intimidate females into copulation,” Nature Communications, 2010) in which a small bar was glued to the back of the female. The bar raised the male up high enough that he couldn’t tap. When the male couldn’t tap, females resisted his advances for much longer periods of time.
I have been making more of a point than ever to get outside for some green time, to be in nature, to do some close and slow looking. By spending more time carefully observing water striders, my curiosity has been piqued and I have learned so much more than I would have with just casual observation. Look up slow looking. It’s something we shouldn’t have to be taught, but the art of sitting still in nature and observing what is going on around you is an art form, one that we can all participate in.